The ever-increasing electro-magnetic susceptibility and lightning requirements in the aircraft industry are forcing controls designers toward more robust solutions to deal with these higher threat levels. These threats include immersion in radio frequency fields with levels as high as 200 volts per meter (V/M) and lightning strike voltages as high as 600 volts (V). One explanation for the increased threat level requirements is that the effectiveness of the shielding of the internal wiring, provided by the skin of the aircraft, has been reduced as more and more of the aircraft's structure is constructed of composite materials. Regardless of the rationale for, or the necessity of these increased requirements, it is becoming obvious that many of the prior art defenses are no longer design options.
Instead of absorbing the energy coupled or injected onto the controller's interconnecting wiring (as is done in the prior art with the use of tranzorbs or metal oxide varistors), it is becoming necessary to present a high impedance to the outside world which is capable of withstanding potentials in the hundreds of volts with respect to the airframe. In many applications such as power driver circuitry, this becomes a problem since the output impedance is purposely kept as low as possible in order to minimize power losses. One method known in the art which may be employed to accomplish both of these goals is to "float" the output driver and its source from the airframe and the chassis of the controller. In this way the impedance of the drive current path may be kept low, and the impedance between the output lines (both drive and return lines) and the structure, to which the threat is referenced, may be kept very high. Isolating the output grounds in this way also acts as a barrier to coupling of the radio frequency noise threats onto the sensitive internal, possibly microprocessor driven, control circuitry, or the radiating of the high frequency noise generated by the digital circuitry out of the controller via the output wiring.
Along with these advantages come a number of design challenges, not the least of which is providing the microprocessor a means to access and measure various system and driver related signals that are referenced to a ground which is isolated from the processing system reference. These signals fall into primarily three categories:
1) Discrete signals such as switch inputs which are easily accommodated through the use of opto-couplers; PA1 2) Alternating current (ac) signals which, in aircraft power generation systems, are primarily 115 volts, 400 hertz, three phase signals which are easily measured by the use of small signal transformers; and PA1 3) Direct current (dc) analog signals.
It is this third category of dc, or slow moving analog (essentially direct current (dc)) signals which presents the most difficulty. The task is to design a multichannel data acquisition system capable of sampling slow moving, positive and negative analog inputs with respect to multiple remote ground references and to store these values in a format accessible to the internal microprocessor-driven control circuitry. The sample may be stored digitally or as an analog value to be read via an analog-to-digital converter. In either case, the remote ground sampling system must be autonomous of the microprocessor system, requiting no synchronization and must allow microprocessor access to the stored values at any instant in time. That is to say, operation of the sampling system, or analog isolation barrier as it will be called hereafter, should be transparent to the microprocessor system. Further, due to the limited amount of available printed wiring board real estate and the ever-increasing economic pressures, the design solution should be of small size and low cost. It is clear that such circuitry could find uses in fields other than aerospace control units, perhaps in medical probes which might demand isolation between equipment and patient or in high voltage power supply applications where a signal to be measured may be referenced to a point several hundred volts above the local ground.
The instant invention is directed to overcoming one or more of the above problems while meeting all of the design requirements.